Personally, I think this problem shows why sane type systems are useful.

In Haskell

    data NestedList a = Single a | Nested [NestedList a]
    -- Or equivalently, type NestedList = Free []

    flatten :: NestedList a -> [a]
    -- This is pretty much the only reasonable definition of flatten the compiler will accept given this type
    flatten (Single a) = [a]
    flatten (Nested xs) = concatMap flatten xs

    -- or given the Free variation of NestedList
    -- flatten = retract